Sahih Muslim
The Book of Prayers (Kitab Al-Salat)
Book Number: 004 / Hadith Number: 1785
Ibn 'Abbas reported Allah's Messenger (may peace be upon him) as saying: Gabriel taught me to recite in one style. I replied to him and kept asking him to give more (styles), till he reached seven modes (of recitation). Ibn Shibab said: It has reached me that these seven styles are essentially one, not differing about what is permitted and what is forbidden.